The present invention relates to a position measuring instrument for absolutely determining the angular position of two objects by means of a first carrier comprising an incremental scale and at least one reference mark and a second carrier comprising an absolute scale and connected to the first carrier by means of a revolution reduction unit.
Angle measuring instruments may be used to measure the relative angular position of two rotatable objects. Typically an angle measuring instrument comprises a housing which encapsulates a photo element and a coded information carrier which is scanned by the photo element. The coded information carrier is mounted on a shaft rotatable with respect to the photo element and housing. The housing is attached to the first object and the shaft is attached to the second object. Thus, rotation of the first object with respect to the second object produces rotation of the information carrier with respect to the photo element. Such measuring instruments are often used on machine tools to measure the relative angular position of various machine tool components.
It is known to use several coded angular calibration scales connected together by means of gear units in angular measuring instruments. This is discussed in German Patent DD-PS No. 89 729. Also, German DE-OS No. 32 43 956 illustrates this type of position measuring device. Such measuring devices comprise individual scanning units for each measuring scale. The position measuring devices are constructed in a multistage manner to increase the measuring range. Furthermore, incremental angular measuring devices are known in which the calibration scale is applied in the form of a concentric lattice onto one carrier. In these devices the measurement is performed by counting the scanned calibrations. The measuring range is practically unlimited since the calibration scale of incremental angular measuring devices is not finite. However, in the event of a system breakdown, the angular position cannot be determined simply by means of a reference impulse derived once per revolution of the reference marks. If the object to be measured is a spindle-driven machine support and a breakdown occurs, the support must be positioned to a location identified in another manner. This type of positioning is described in KEM Periodical, October 1983, page 109. However, these positioning methods can be complicated and time consuming.